[libomptarget][nfc] Introduce atomic wrapper function

Summary:
[libomptarget][nfc] Introduce atomic wrapper function

Wraps atomic functions in a template prefixed __kmpc_atomic that
dispatches to cuda or hip atomic functions. Intended to be easily extended
to dispatch to OpenCL or C++ atomics for a third target.

Reviewers: ABataev, jdoerfert, grokos

Reviewed By: jdoerfert

Subscribers: Anastasia, jvesely, mgrang, dexonsmith, llvm-commits, mgorny, jfb, openmp-commits

Tags: #openmp, #llvm

Differential Revision: https://reviews.llvm.org/D71404